W. K. BEOOKS ON THE GENUS SALPA. 193 



the ancestors of the vertebrates must have had at that time another 

 mouth, and during the long series of stages of development, while the 

 gill-slits were gradually assuming the function of a mouth, food must 

 have been taken in through both openings ; for the new function of the 

 gill-slits must have been acquired slowly alongside their old function, 

 until the new mouth finally became so perfectly adapted for its new 

 function that it supplanted and replaced the old one. 



According to Dohrn, these considerations force us to believe that 

 the primitive mouth of the ancestors of the vertebrates and of the tuni- 

 cates was situated in the fossa rhomboidea, where an oesophagus pushed 

 inwards to join the mid-gut, in the same way that it is joined in insect 

 embryos by the fore-gut. This primitive mouth and its oesophagus were 

 homologous with the corresponding organs of modern arthropods and 

 annelids. The mouth of the modern vertebrates is then to be regarded 

 as a secondary mouth, which has gradually supplanted and replaced the 

 old one on account of its greater efficiency. 



It follows from this, according to Dohrn (p. 56), that the "so-called 

 larva" of the ascidians is a degenerated fish, and that all the features 

 which show the derivation of the cyclostomes from the fishes show also 

 that the process of degeneration has reached its extreme in the tunicates. 

 The cyclostomes are held to owe their degeneracy to parasitism, and the 

 most important element in the more advanced process of degeneration is 

 that the ascidians no longer fasten themselves to fishes nor make use of 

 their bodies as food, but that they fasten themselves to stones, to ships, 

 or to the bodies of other animals which do not serve as food, such as the 

 shells of crabs or the tubes of annelids. 



The mouth (p. 57) which in the cyclostomes serves both as an organ 

 for attachment to the skin of fishes, and also as a sucker for extracting 

 their blood, has become converted in the ascidians into an organ for 

 attachment; and these animals have thus lost their old mouth, which 

 was homologous with that of the true vertebrates, and have acquired a 

 new one which is homologous with the vertebrate nasal chamber. 



The process, Dohrn says, must be represented as follows : The fishes 

 take in the water for respiration through the mouth, but as this is used 

 by the parasitic cyclostomes as a sucker, they have acquired another 

 arrangement, and the water is not only discharged through the gill-slits, 

 but is also inhaled through them, and, in the myxenoids, through the 

 nasal passage also, which has in the tunicates become the functional 

 mouth. The vertebrate mouth has lost its old function in the cycle- 



